Members of the Division of Comparative Neurobiology of the Marine Biomedical Institute are examining the organization of the nervous systems of several invertebrate and vertebrate preparations. Four projects are concerned with the marine mollusc, Aplysia. The neural basis of simple reflexes in this animal are being studied using chronically implanted electrodes and a computer analysis of spike trains generated during behavior. Neuroendocrine control of egg laying is examined in behavioral experiments. A neurochemical analysis is directed at extraction, identification and synthesis of peptide hormones from the atrial gland, and the sequence of activation of the neuroendocrine bag cells is being investigated. Certain identified neurons in Aplysia contain glycine, and now an effort will be made to measure the release of glycine; the possibility that the same neurons utilize a peptide as a chemical messenger will also be examined. The remaining four projects involve vertebrate preparations. The neural system controlling locomotion in the stingray is being analyzed. Pain transmission pathways are being studied in the monkey, as well as descending control systems that may be responsible for mediating analgesia. A systematic investigation is being done of the anatomic organization of descending monoaminergic and peptidergic pathways in the monkey. Finally, the organization of unmyelinated fiber systems in the mammalian peripheral nervous system and spinal cord is being examined.